In these gear boxes, the reduction elements, comprising gears and bearings, are generally lubricated by pressurized oil through one or more lubrication pumps which are driven by sets of gears taking their movement from a main train of the reduction gears, or from an auxiliary drive source.
When the normal lubricating system of the main reduction unit is defective, either by loss of oil due to a leak, or by deterioration of the pressure generator or of the pump, it is obviously important for a minimum lubricating system to be able to replace it. Thus, a rotary-wing aircraft could, if needed, reach a favourable landing point with minimum power. This would be particularly advantageous for rotary-wing aircraft making extensive flights over the sea, for example those used for "offshore" oil working.
Emergency lubricating systems already exist which are adapted for vehicles or boats and which consist in providing at least one auxiliary oil reservoir, disposed at a higher point than the mechanisms to be lubricated. For example, in the U.S. Pat. No. 4,721,185 an oil reserve system for vehicles is described in which a complementary reservoir fed with oil supplies filtered oil, drawn by a pump from the hydraulic system of the vehicle. To take the oil from the reservoir a pump is therefore required which is not safe from defective operation. In another field, such as that of boats, as shown by the example in the U.S. Pat. No. 2,988,154, a normal lubricating circuit is adapted to feed a second reservoir placed in a high situation, with pumps and valves for maintaining an equal oil pressure whatever the condition of use.
All these systems are emergency lubricating systems for palliating a partial or total pressure failure and they use the distribution of the normal device to provide equivalent lubrication. But such an emergency system remains inoperative if the normal circuit is itself totally defective, for example following the total loss of lubricating fluid. This is why the applicant has designed a device which ensures minimum lubrication of the essential elements of the reduction unit, even if there is an accidental absence of supply for the normal circuit and the auxiliary emergency circuit, for pressurized lubrication of said reduction unit, including when the normal oil reserve which feeds the normal lubricating circuit is exhausted. The device in question makes minimum lubrication then possible so that the reduction unit can operate for a sufficient time to allow landing under good safety conditions.